In optical fibers, a supercontinuum light is formed when a collection of nonlinear processes act together upon feeding of a pump beam in order to cause spectral broadening of the original pump beam. The result may be a smooth spectral continuum spreading such as over more than an octave of wavelengths. Suitable non-linear processes are for example self-phase and cross-phase modulation, four-wave mixing, Raman gain or soliton based dynamics, interacting together to generate the supercontinuum light. In order to get the broadest continua in an optical fiber, it is most efficient to pump in the anomalous dispersion regime; however a spectral continuum may in some situations be obtained by pumping in the normal dispersion regime. Microstructured optical fibers are suitable for supercontinuum generation due to their high non-linearity and their customizable zero dispersion wavelength.
The term “microstructured fibers” in this context is meant to cover fibers comprising microstructures such as photonic crystal fibers, photonic bandgap fibers, leaky channel fibers, holey fibers, etc. Unless otherwise noted, the refractive index refers to the average refractive index which is usually calculated separately for the core and each cladding layer surrounding it, whether the fiber is a standard fiber, where the core and any cladding layers surrounding that core have a substantially homogeneous refractive index, or a microstructured fiber where the core and/or one or more cladding layers comprise microstructures. A cladding layer is defined a layer with a thickness and surrounding the core where the refractive index is substantially homogeneous or where the where the layer has a base material with substantially homogeneous refractive index and a plurality of microstructures arranged in a uniform pattern.
A problem in relation to the present optical fibers for supercontinuum generation is that high optical peak power and/or pulse energies over time damage(s) the optical fibers. The article “Analysis of the scalability of single-mode near-infrared supercontinuum to high average power” by Rui Song et al, published 29 Jan. 2013 in IOP Publishing, Journal of Optics, J. Opt. 15 (2013) 035203 examines the restrictions imposed by thermal and nonlinear effects, fiber end facet damage, pump and fiber combiner limits, and damage to the amplifier fibers. In terms of nonlinear effects, the restriction is the self-focus effect, whose threshold is around 4 MW. The fiber end facet damage threshold limits the power density to 10 W μm−2, but the damage threshold can be improved by using an end-cap with a large diameter. Here the end-cap consists of pure fused silica. By expanding the fiber mode in the end-cap, the power density on the surface can be reduced to under the average output power, so that the surface damage threshold is increased.
Supercontinuum generation is a complex process, and any quantitative explanation of the underlying physics must take into account a number of different fiber and pulse parameters. Nonetheless, it is generally accepted that the most efficient method to obtain a very broad supercontinuum is by using a pump wavelength slightly in the anomalous group-velocity dispersion (GVD) regime of a highly nonlinear Photonic Crystal Fiber (PCF) with only one zero-dispersion wavelength (ZDW) below the absorption limit of the material. In contrast pumping in the normal GVD regime of a PCF will in general reduce the bandwidth and require a longer length of the PCF (J. Dudley et al, “Supercontinuum generation in photonic crystal fiber”, Reviews of Modern Physics, Vol. 78, p. 1135, October-December 2006).
Normally high power supercontinuum sources use a pump wavelength of around 1064 nm and a PCF with a core size of about 3.5 to 5 μm. A standard calculation of the dispersion shows that the core size of a PCF having ZDW of 1064 nm increases with the hole size, and for very large hole sizes it reaches about 6 μm. Hence in order to have anomalous dispersion at 1064 nm in a PCF the core size is limited to around 6 μm or less.
E.g. K. K. Chen report a “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power” (Optics Express, Vol. 18, No. 6, p. 5431, 15 Mar. 2010). They used a 21 ps 1060 nm laser which is pumped into a 2 m long PCF with a core size of 4.4 μm and a ZDW of 1012 nm and found that the maximum average power was limited by thermal damage. Hence they concluded that power scaling might therefore be possible by using a mode-expanding end-cap at the input to the fiber.
G. Genty et al report normal pumped supercontinuum in PCFs with cores up to 20 μm in the paper “Supercontinuum generation in large mode-area microstructured fibers” (Optics Express, Vol. 13, No. 21, p. 8625, 17 Oct. 2005). Here it is shown that a supercontinuum spanning more than an octave can be generated by a 3 ns 1064 nm pump in a 100 m long fiber with a 10 μm core, and the mechanisms leading to the continuum in this case primarily rely on the processes of cascaded stimulated Raman scattering and four-wave mixing. It is concluded that the large area of the fibers should allow for the generation of extremely high power supercontinuum as the damage threshold is considerably increased. However, it is observed that the normalized intensity is significantly higher above than below the pump wavelength of 1060 nm, and that a further increase of the core size leads to a significant decrease of the power on the blue side of the spectrum.
C. Xiong et al have reported another example of normal pumped supercontinuum in “Enhanced visible continuum generation from a microchip 1064 nm laser” (Optics Express, Vol. 14, No. 13, p. 6188, 26 Jun. 2006). Here a tapered fiber approach was used, the first fiber section has a core size of 5 μm and a ZDW of 1103 nm. The proximity of the ZDW to pump wavelength allows for a strong four-wave mixing gain to 742 nm, allowing >35% conversion of the 1064 nm pump light over a 3 m long fibre. The fiber is tapered to a core size of 1.7 μm giving a ZDW of 700 nm, as previous studies have shown that small-core PCF are ideal for super continuum generation from pulsed sources at wavelengths from 600 to 800. This fiber gives a bright single mode visible light source with output power of up to 20 dB/m.